It is prevented the overvoltage-damages on the equipments due to a neutral line loss.
In case of the circuit of single-phase, three-wire and earthed mid point; an overvoltage occurs in the equipments of the circuit if a neutral conductor does loss . These breakers will be tripped by this overvoltage.
This kind of breaker is MCCB incorporating neutral pole protection.
This kind of breaker is ELCB incorporating neutral pole protection.
This kind of breaker is the earth leakage alarm breaker incorporating neutral pole protection.